CMOS Compatible Microchannel Heat Sink for Electronic Cooling and Its Fabrication

ABSTRACT

The present invention is a CMOS compatible polymer microchannel heat sink for electronic cooling applications. The heat sink can be fabricated directly on the chip surface with an insulation layer by standard polymer surface micromachining techniques. The heat sink device comprises a thin insulation layer on the chip surface, a thin-film metal layer as bottom surface, metal side walls, and a polymer top wall over the microchannels and inlet-outlet reservoirs.

TECHNICAL FIELD OF INVENTION

The present invention is a microchannel heat sink for electronic coolingapplications. The heat sink is fabricated using CMOS compatiblematerials and surface micromachining techniques.

BACKGROUND OF INNOVATION

With the advances in microelectronics technology, it is now possible tofit high performance, high speed analog and digital circuits of varyingpurpose into very small volumes. In contemporary integrated circuits(IC), millions of transistors can be fitted into an area of 1 mm²,hence, faster and more functional chips may be produced. On the otherhand high density integrated circuits require more power. Therefore,considerable amount of heat is generated in compact volumes and itsremoval from the system becomes a major design problem. The increase inlocal temperatures may cause decreased performance or significantdamages to the circuits unless the generated heat is removed timely.

Current commercial cooling methods for electronic applications are basicnatural and forced air cooling, liquid cooling systems and somerefrigeration based systems. The most widely used method is forcedconvection air cooling, but heat transfer capacity of air cooling islimited about heat flux values of 60 W/cm² and modern CPUs are rapidlyapproaching this limit [1]. Moreover, hot spots on the CPU die causeeven higher heat fluxes, about 3 to 8 times more of the average heatflux of the die, at local points on the chip surface [2]. Liquid coolingsystems and refrigeration solutions can cope with heat fluxes up to 100W/cm², but those systems are bulkier and more complex compared with aircooling systems, increasing the overall cost and causing reliabilityproblems [3]. The use of nanofluids, suspensions of nanoparticles inbase fluids, as the working fluid yields an enhancement in heat transferof more than 50% [4] compared to the use of the base fluid only.

In order to remove higher heat fluxes from the electronic chip surfaces,several new techniques have been introduced. Among them microchannelheat sinks have attracted significant attention for the last threedecades [5]. Microchannel heat sinks are basically very compact heatexchangers with enormous area to volume ratios. This ratio provides avery large heat transfer area over the chip surface therefore the heatremoval rates of those heat exchangers are much higher than their macroscale versions. Moreover, the small hydraulic diameters of microchannelsprovide higher heat transfer coefficients, increasing the heat removalrate even further. There are various fabrication methods formanufacturing microchannel heat sinks such as, micromilling [6], siliconetching [7], or metal electroplating [8].

Micromilling process requires thick metal substrates and the heat sinkis manufactured separately from the microchip. Both factors increase theoverall thermal resistance of the cooling system. In monolithic designsthe heat sink is manufactured along with the circuit by usingmicrofabrication techniques such as silicon etching and electroplating.This provides lower thermal resistance since the junction-to-fluiddistance is much shorter compared to that in separate heat sinks.However silicon etching processes have some drawbacks such as thedifficulty of back side etching of finished CMOS wafers which may causereduced fabrication yield [9], or the need for a design change of thecircuit to create area for coolant inlet and outlets [7]. On the otherhand, electroplated channels can be fabricated directly on top of thecircuits without a need for a design change of the underneath circuitry.

Joo et al. investigated electroplated metal microchannels for electroniccooling applications [8]. Their electroplated, radially divergingmicrochannel heat sinks were able to extract 35 W/cm² with air as thecoolant. In this heat sink the top of the channels were covered with theelectroplated overhangs extended from the sidewalls, therefore thelargest channel width was limited to fewer than 50 microns. On the otherhand, the channel height was defined by sacrificial layer thicknesswhich is 70 microns at most. Microchannels with such small dimensionsand hydraulic diameters cause enormous pressure drop when used withliquid coolants, therefore the working fluid should be a gas such asair. Since heat transfer coefficient in air cooling is quite lowcompared to liquid cooling, the maximum heat removal capacity of thosemicrochannel heat sinks were expected to be limited less than 50 W/cm².

In order to produce a CMOS compatible surface micromachined microchannelheat sink, the present invention makes use of a polymer top wall insteadof electroplated overhangs for covering the microchannels. This designenables fabrication of microchannels that are several hundred micronswide. Therefore higher hydraulic diameter microchannels can befabricated as well as smeller ones. Besides polymer coating is a simpleand CMOS compatible low temperature process. Based on the availablepumping power and cooling requirements of the electronic chips, theproposed metal-polymer microchannel heat sinks can be used with variouscoolants, including gaseous fluids like air, refrigerant, liquidcoolants like water, ethylene glycol, or liquid-solid suspensions likenanofluids. With this flexibility, it is possible to obtain much higherheat removal rates from the chip surface.

The Aims of the Invention

With the proposed invention of CMOS compatible surface micromachinedmetal-polymer microchannel heat sink, it is aimed to obtain a high heattransfer capacity, light weight, monolithic heat sink structure whichcan be fabricated easily with standard CMOS compatible surfacemicromachining techniques and can be operated with gaseous or liquidcoolants, or nanofluids.

Compared with the current commercial cooling technologies, theadvantages of the proposed device can be summarized as;

-   -   High area to volume ratio of microchannels increases the overall        heat transfer capacity of the heat sink.    -   The microchannels can be fabricated on the same wafer as the        chip itself.    -   Fabrication on the same wafer decreases the distance between        heated chip surface and coolant medium, decreasing the thermal        resistances in the cooling system significantly.    -   Using high performance microchannels in the heat sink enables        very compact cooling solutions which can be utilized in mobile        computer systems as well.

Such advantages are common to most of the microchannel heat sink devicesstudied in the literature before. In addition to these, the proposedinvention adds further advantages to similar microchannel heat sinkdesigns, such as;

-   -   The microchannels can be fabricated directly on top of the        heated chip surface by using CMOS compatible surface        micromachining techniques, instead of silicon etching processes.    -   Since the heat sink can be fabricated directly on the chip        surface a monolithic cooling system is obtained.    -   No design change required on the chip since the microchannels        are fabricated over the chip surface with post-CMOS surface        micromachining techniques.    -   Microchannels with different cross sections and hydraulic        diameters can be fabricated, thus enabling usage of liquids and        nanofluids as well as gases as coolants, with acceptable pumping        power requirements.

DESCRIPTIONS OF FIGURES

Several figures of the different parts of the polymer microchannel heatsink assembly are given in the appendix to provide easy understanding.Brief descriptions of the figures are as follows;

FIG. 1 Perspective view of microchannels on top of electronic chip withmicrochannel walls exposed.

FIG. 2 Top view of microchannel heat sink with polymer top wall removed.

FIG. 3 Cross sectional view of the microchannels along flow direction.

FIG. 4 Fabrication flow of microchannel heat sink.

DESCRIPTION OF THE PARTS

Composing parts of the polymer microchannel heat sink are shown in thefigures by numbers and the respective names are provided below.

-   1—Silicon wafer under the microelectronic circuit.-   2—Microelectronic circuit with electrical insulation layer.-   3—Microchannel side walls.-   4—Inlet reservoir.-   5—Outer walls of microchannel heat sink.-   6—Polymer top wall.-   7—Outlet reservoir.-   8—Metal seed layer.-   9—Photoresist Layer-   10—Thin Polymer Layers

DETAILED DESCRIPTION OF THE INVENTION

The present invention is composed of mainly four elements namely;

-   -   Metal seed layer (8) on top of the microelectronic circuit with        electrical insulation layer (2),    -   Parallel microchannels with metal side walls (3),    -   Polymer top wall (6),    -   Inlet (4)-outlet (7) reservoirs.

The electrical insulation layer is deposited on the microelectroniccircuit with electrical insulation layer (2) by means of standard vapordeposition techniques, in order to prevent any electrical interactionbetween the electronic chip and the coolant. The insulation layer can bemade of either silicon dioxide, silicon nitride or a polymer film.Microchannels lay on top of the metal seed layer (8) which is depositedover the insulation layer. Microchannels are formed by metal seed layer(8) at the bottom wall and electroplated metal (e.g. Copper, Nickel orGold) on the microchannel side walls (3) and polymer top wall (6). Thinpolymer layers can be coated on inner surface of the microchannels,including the metal surface on the microchannel side walls (3), metalseed layer (8) and polymer top wall (6) as the final step in thefabrication process to prevent corrosion due to contact of coolant andmetal.

Fabrication method of the microchannel heat sink is shown in FIG. 4schematically. Fabrication is a two-mask process. Microchannel heat sinkis fabricated directly on top of the microelectronic circuit withelectrical insulation layer (2). First a thin metal seed layer is coatedon top of the insulation layer for electroplating process (FIG. 4-a).Seed layer is patterned by using Mask-1 with UV-lithography and wetmetal etching, in order to prevent electroplating on top of contact padsof the underneath circuitry. By using Mask-2 a sacrificial thickphotoresist layer is patterned, forming the microchannels andinlet-outlet reservoirs (FIG. 4-b). Microchannel dimensions are definedby Mask-2 layout and photoresist thickness. Then metal is electroplatedbetween the sacrificial photoresist lines (FIG. 4-c). A thick polymerlayer is then coated forming the polymer top wall (6) (FIG. 4-d). Thislayer provides both visibility and easy sealing. Next, the inlet andoutlet connection ports and contact pads are opened in O₂ plasma with athick photoresist layer patterned by again Mask-1, protecting thepolymer top wall over microchannels. Finally, the sacrificialphotoresist is removed by acetone-IPA-methanol release (FIG. 4-e).

By using different photoresists, various microchannel thicknesses can beobtained. Moreover, employing the polymer top wall (6), not only narrowmicrochannels, but also larger channels, as wide as several hundredmicrons can be fabricated. As a result of dimension flexibility,different working fluids (gas, liquid, nanofluid) can be used ascoolants with acceptable pressure drops.

The overall electronic cooling system is composed of the metal-polymermicrochannel heat sink (FIG. 1), inlet and outlet microfluidicinterconnection ports (not shown), piping (not shown), pump/compressor(not shown) and external heat exchanger (not shown). Parts other thanthe microchannel heat sink are standard cooling system components whichare already commercially available.

Operation of the System

The metal-polymer microchannel heat sink provides cooling by means ofsteady state flow of a working fluid inside the microchannels. Theworking fluid can be either water or some other coolant, based on theoperation conditions and cooling requirements of the chip. The workingfluid is pumped in the closed loop cooling system by a pump. Then thefluid flows through the pipes and reaches the inlet reservoir (4) of thepolymer microchannel heat sink. In the inlet reservoir the working fluidis distributed to microchannels and flows through microchannels. Duringthe flow the working fluid is heated by the heat flux coming from themicroelectronic circuit with electrical insulation layer (2) under themetal seed layer (8). The temperature of the working fluid increaseswhile flowing down the microchannels. The hot working fluid is collectedin the outlet reservoir (7) of the heat sink and leaves the microchannelheat sink through the microfluidic interconnection ports.

The hot working fluid then flows through pipes and radiator (not shown)where it is cooled by ambient air by means of natural or forcedconvection. The cooled down working fluid flows thorough pipes andreaches pump (not shown) completing the one cooling cycle.

REFERENCES

-   [1] Wei, Jie (2008) Challenges in Cooling Design of CPU Packages for    High-Performance Servers, Heat Transfer Engineering, 29: 2, 178-187-   [2] Mahajan R., (2006), Cooling a Microprocessor Chip, Proceedings    of the IEEE, 94: 8.-   [3] Kandlikar S. G., Grande W., (2004), Evaluation of Single Phase    Flow in Microchannels for High Heat Flux Chip    Cooling—Thermohydraulic Performance Enhancement and Fabrication    Technology, Heat Transfer Engineering, 25: 8, 5-16-   [4] L. Godson, B. Raja, D. Mohan Lal, and S. Wongwises, “Enhancement    of heat transfer using nanofluid—An overview”, Renewable and    Sustainable Energy Reviews 14 (2010) 629-641.-   [5] D. B. Tuckerman, R. F. W. Pease. “Method and means for improved    heat removal in compact semiconductor integrated circuits and    similar devices utilizing coolant chambers and microscopic channels”    U.S. Pat. No. 4,450,472, May 22, 1984-   [6] P. S. Lee, S. V. Garimella, and D. Liu, Investigation of heat    transfer in rectangular microchannels, International Journal of Heat    and Mass Transfer, vol. 48, 2005, pp. 1688-1704.

[7] G. Kaltsas, D. N. Pagonis, and A. G. Nassiopoulou, Planar CMOScompatible process for the fabrication of buried microchannels insilicon, using porous-silicon technology, Journal ofMicroelectromechanical Systems, vol. 12, 2003, pp. 863-872

-   [8] Y. Joo, H. C. Yeh, K. Dieu, and C. J. Kim, Air cooling of a    microelectronic chip, Journal of Micromechanics and    Microengineering, vol. 18, 2008, p. 115022-   [9] R. J. Bezama, E. G. Colgan, J. H. Magerlein, R. R. Schmidt.    “Apparatus and methods for microchannel cooling of semiconductor    integrated circuit packages” U.S. Pat. No. 7,139,172, Nov. 21, 2006

1-5. (canceled)
 6. A method of fabricating microchannel heat sink for electronic cooling applications with liquid, nanofluid and gas, the method comprising the steps of: electroplating a plurality of microchannel side walls on a thin-film metal seed layer between a sacrificial photoresist layer; removing the sacrificial photoresist layer by acetone-IPA-methanol release; wherein the sacrificial photoresist layer and the plurality of microchannel side walls are covered with a polymer top wall before the sacrificial photoresist layer is removed by acetone-IPA-methanol release.
 7. The method of claim 6 further comprising: coating a plurality of thin polymer layers on the inner surface of the plurality of microchannels, including the metal surface on the polymer top wall after the sacrificial photoresist layer is removed by acetone-IPA-methanol release.
 8. A micro-channel heat sink comprising: a plurality of metal side walls; a thin-film metal seed layer; a plurality of sacrificial photoresist layers; and a polymer top wall; wherein the plurality of photoresist layers and the plurality of metal side walls are covered by the polymer top wall.
 9. The micro-channel heat sink of claim 8 wherein the inner surface of the micro-channels including the metal surface on the micro-channel side walls metal seed layer and polymer top wall are coated with thin polymer layers.
 10. The micro-channel heat sink of claim 8 wherein cooling is supplied by circulation of a fluid through micro-channels.
 11. The micro-channel heat sink of claim 8 wherein a cooling material is selected from a group consisting of liquid, nanofluid and gas. 